1. Field of the Invention
The present invention relates to gas leak alarm and location generally, and more specifically, the invention is directed to an apparatus and method for leak-checking of gas or liquid-tight components on a production line or in the field.
2. Discussion of the Prior Art
The leak testing of compressors, heat exchangers, fuel tanks, fuel and hydraulic lines, pressure vessels, and window and door seals, etc., is an important manufacturing consideration in many different industries. In many cases, the gas-tight or liquid tight integrity of these components and/or systems is usually determined by some form of a pressure-decay test. With this technique, the unit under test is injected with air to some specified overpressure, and the pressure is monitored for a specified time period. If the pressure does not decay below a specified value at the end of the designated time period, the component under test is considered to be leak-free. This is a very simple, cost-effective leak checking method, and it is used for production line leak checking whenever possible. However, the pressure-hold method is essentially a yes/no leak tight test in that it only indicates to the operator whether or not the unit has a leak--it does not tell him the leak location. Furthermore, for large components with small leaks, a lengthy time period is required. The procedure is also affected by any temperature changes which may occur during the monitoring period.
A more sensitive technique involves drawing a vacuum on the component and then completely surrounding it with helium gas. A detector inside the vacuum system notifies the operator if helium is present in the air being pumped from the component. This technique is capable of detecting leaks as small as 10.sup.-9 scc/sec, but is very expensive to set-up and to maintain, and as with the pressure-decay technique, does not indicate the location of the leak.
Generally, components which fail the pressure-decay or helium leak tests are rejected from the production process and submitted to some form of leak location testing. Pressurization/immersion, pressurization/soaping, ammonia-sensitive paint and tracer gas injection/detection, are the most common industrial techniques currently being used to pinpoint leak sources.
The pressurization/immersion technique consists of pressurizing the component, totally immersing it in water or some other clear liquid, and observing the point of bubble emergence. This technique works quite nicely in situations involving small components which are not adversely affected by liquid immersion. However, the technique does usually require some post-test clean-up and/or drying procedure. This technique is capable of locating leaks as small as 10.sup.-4 scc/sec with proper lighting, use of low surface tension liquids, and if adequate viewing time is allowed. It is a very labor intensive, time consuming method which requires extreme worker concentration for long periods of time. It is a leak location technique which does not very easily lend itself to automation.
Pressurization/soaping is another leak location technique which is generally used to locate leaks from components or larger complex systems where total immersion is not practical. In this technique, the leaky component is pressurized with air, painted or sprayed with a thin viscous liquid (usually soap), and observed for the presence of bubbles which indicate the leak location. This technique requires that the liquid soap be placed on the leak, and observed for bubble formation before it either evaporates or flows away. It is somewhat more labor intensive technique than pressurization/immersion and always requires post-test clean-up. Experienced technicians say they can locate leaks as small as 10.sup.-3 scc/sec with this technique, making it about 10 times less sensitive than the pressurization/submersion technique.
With the pressurization/ammonia-sensitive painting technique, the component is coated with a water soluble, ammonia-sensitive paint, a small amount of liquid ammonia is injected into the component, it is sealed and pressurized with air. The ammonia/air mixture emerging from the leak produces a discoloring of the special paint, thus pin-pointing the location of the leak. This technique is quite expensive, involves the use of a toxic material (ammonia), and requires extensive post-test cleanup. However, it offers complete coverage of the component and is quite sensitive. According to the paint manufacturer, an observer can see paint discoloration within one minute at a distance of 5 meters produced by a 10 micron diameter pinhole leak pressurized to 1.3 atm (5 psig). Under the same conditions, a 30 micron pinhole leak will produce a 6 mm diameter discoloration within one minute. These leak rates are estimated to be in the 10.sup.-3 scc/sec range.
The tracer gas injection/detection technique involves pressurizing the component with a tracer gas, usually helium (He) or a chlorofluorocarbon (CFC), and surveying the exterior with a sensitive sniffer-type detector. This technique is extremely sensitive, capable of locating leaks as small as 10.sup.-6 scc/sec if the intake of the sniffer is placed directly over the source of the leak. Drawbacks to the He approach are the cost of the gas and the detection system; however, this technique is relatively free of background gas false readings. On the other hand, the cost of the CFC gas and detectors is quite reasonable, but these sniffers are affected by a host of common background gases, and are currently being phased out as tracer gases for their adverse affect on the environment. Furthermore, with this technique, location of small leaks can be masked by the presence of a large leak located nearby.
The instant invention involves a physical process commonly known as the photo-acoustic effect, which is used in various forms as a gas detection technique. For example, U.S. Pat. No. 4,516,858 to Gelbwachs, describes an apparatus in which a laser beam is distributed to a number of photo-acoustic cells via fiber optic cables for the purpose of monitoring hazardous gas concentrations at multiple sites.
U.S. Pat. No. 4,557,603 to Oehler et al, discloses an apparatus for the selective detection of a variety of gases using the photo-acoustic effect. In this case, a monochromator is used to vary the wavelength of the light introduced to a photo-acoustic cell which contains the gas to be analyzed.
U.S. Pat. No. 4,622,845 to Ryan, et al, discloses an apparatus using a pulsed infrared light source and an acousto-optic tunable filter to provide illumination of a photo-acoustic cell containing a gas sample extracted from the environment.
In all of the above inventions, the gas to be detected must be introduced into a photo-acoustic cell which is then irradiated with pulsed or modulated light which is spectrally selected to be strongly absorbed by the gas within the cell. The purpose of all of the above mentioned inventions is for concentration measurements of the gases of interest.
In an article entitled "Photo-acoustic detection and ranging a new technique for the remote detection of gases", Brassington, J. Phys. D: Appl. Phys., volume 15, page 219, 1982, an apparatus is described for determining the presence and range to a gas source using a pulsed laser and a microphone detector. The distance to the gas source, or range along the line-of-sight of the pulsed laser beam, is determined from the delay in receiving the acoustic pulse generated when the laser light is absorbed by the gas of interest. This technique requires a pulsed laser, and is not capable of rapidly determining the precise source of gas leaks.